1. Field of the Invention
This invention pertains to apparatus and methods for transferring heavy loads between first and second structures one of which is floating on heaving water and, more particularly, to a package suitable for use with a standard floating crane to convert the crane to one which has a heave compensating capability and an accurate fine positioning control of the load.
2. Description of the Prior Art
Various types of heave compensating devices have been used heretofor for enabling the transfer of loads from one structure to another where at least one of the structures is floating and subject to vertical displacement due to wave action. The problem of heave action becomes particularly severe as the size of the load increases, such as where a complete drilling or oilwell production module is to be transferred between a floating barge and a stationary offshore platform.
Some of the prior art devices have gone toward electronic synchronization of relative movements between the load and the stationary structure whereas others have employed heave compensating cylinders in some fashion for holding the load vertically stationary relative to the stationary structure during the loading and unloading of the load onto or off of the stationary structure. Typical examples of the latter devices are illustrated in the U.S. Pat. Nos. 2,817,212 and 2,907,172. The latter patent, in particular, also employs a hauldown system which pulls the load onto the stationary structure while overcoming the pressure in the heave compensating cylinders to provide a fine control of moving the load relative to the stationary structure.
The difficulty with the known prior art devices are their inherent costs and complicated nature. For example, in U.S. Pat. No. 2,907,172, a specially constructed barge is required for handling the heave compensating and hauldown system.
A second difficulty lies in the fact that the load is not only subjected to vertical motion relative to the stationary structure but in addition is subjected to horizontal lateral and horizontal rotational motion.